1. Field of the Invention
The present invention relates to a battery state monitoring circuit for controlling charge/discharge of a secondary battery, and a battery device including the battery state monitoring circuit.
2. Description of the Related Art
A battery state monitoring circuit includes an overcurrent detection circuit for protecting a battery device from an abnormal current, which flows when a load has a problem. Various inventions are disclosed for the overcurrent detection circuit for protecting the battery device (see, for example, Japanese Patent Application Laid-open No. Hei 10-224997).
FIG. 3 illustrates a conventional battery device including a battery state monitoring circuit. A conventional battery device 20 includes a secondary battery 1, a switch circuit 3, a sense resistor 4, and a battery state monitoring circuit 21. The battery state monitoring circuit 21 includes a first reference voltage circuit 5, a second reference voltage circuit 6, a first voltage comparator circuit 7, a second voltage comparator circuit 8, a first PMOS transistor 9, a second PMOS transistor 10, a first current source 14, a second current source 15, a capacitor 13, and a control circuit 2.
The first voltage comparator circuit 7 compares a voltage of the first reference voltage circuit 5 and a voltage of the sense resistor 4 so as to detect an overcurrent. The second voltage comparator circuit 8 compares a voltage of the second reference voltage circuit 6 and the voltage of the sense resistor 4 so as to detect an overcurrent. Here, the voltage of the first reference voltage circuit 5 is lower than the voltage of the second reference voltage circuit 6. The first PMOS transistor 9, the second PMOS transistor 10, the first current source 14, the second current source 15, and the capacitor 13 together form a delay circuit. The control circuit 2 receives a detected signal from the delay circuit and controls the switch circuit 3.
The above-mentioned battery state monitoring circuit 21 operates as follows to function to protect the battery device 20 from an overcurrent.
If a load connected between external terminals +VO and −VO of the battery device 20 has a problem and an abnormal current flows, the voltage across the sense resistor 4 is increased. Then, if the voltage of the sense resistor 4 exceeds the voltage of the first reference voltage circuit 5, the first voltage comparator circuit 7 detects an overcurrent and outputs a signal of L. The first PMOS transistor 9 is turned ON, and accordingly the capacitor 13 is charged with a current of the first current source 14. The control circuit 2 receives a detected signal from the delay circuit after a predetermined delay time period. Then, the control circuit 2 turns OFF the switch circuit 3 to interrupt a current path.
If the load becomes a short-circuit state and a larger amount of current flows, a voltage of the secondary battery 1 is abruptly decreased. On this occasion, if the voltage of the sense resistor 4 exceeds the voltage of the second reference voltage circuit 6, the second voltage comparator circuit 8 detects an overcurrent under the short-circuit state and outputs a signal of L. Therefore, the capacitor 13 is charged with the current of the first current source 14 and a current of the second current source 15. In other words, the delay time period is shortened so that the control circuit 2 may turn OFF the switch circuit 3 more rapidly.
However, if the secondary battery 1 has not been charged sufficiently or the secondary battery 1 has degraded because of the life, the voltage of the secondary battery 1 is abruptly decreased by the overcurrent even before the load becomes the short-circuit state. In this case, the conventional battery state monitoring circuit 21 has a problem that the second voltage comparator circuit 8 does not detect the overcurrent under the load short-circuit state and hence a desired delay time period cannot be obtained.